1. Field of the Invention
The present invention generally relates to an auto focus (AF) device of a mobile communication apparatus, and more particularly to an auto focus device which can prevent image quality from being degraded due to drop test of a floating device of a voice coil motor of the auto focus device.
2. Description of Related Art
Electronic products such as mobile phones, digital cameras and digital video cameras are usually equipped with voice coil motors as driving devices. Particularly, the voice coil motor is one of the key components of a high end mobile phone with an auto focus camera. Conventional voice coil motor is constituted by a floating device, a winding coil, a magnet and a lens located inside the floating device. The auto focus function is performed through the movement of the floating device inside the voice coil motor. The floating device moves due to the interaction between the magnet and the winding coil when an electric current flows through the winding coil since the magnet surrounds the magnet.
However, some problems resulting from the floating device are presented in electronic products which use conventional voice coil motors during test and use. Since the floating device is under controlled only if an electric current flows through the winding coil, the floating device will randomly move without being controlled inside the voice coil motor when the magnet and the winding coil do not interact with each other. For example, when a high end mobile phone with an auto focus camera is under a drop test, the floating device inside the voice coil motor will irregularly vibrates and collides with the inner surface of voice coil motor to generate particles. These particles will pass through the gap between the floating device and the inner surface of voice coil motor and fall on the surface of image sensors with a large possibility during further vibration. Thus black dots or shadow regions will present on images and degrade the quality of the images.
FIGS. 1A and 1B show schematic views of a floating device of a conventional voice coil motor moving and vibrating to generate particles. FIG. 1A shows that a floating device 102 inside a voice coil motor 100 moves to perform auto focus on an image sensor 104 due to the interaction between a magnet (not shown) and a winding coil (not shown) when an electric current flows through the winding coil. FIG. 1B shows that the floating device 102 irregularly vibrates (up or down) and collides (slightly toward right or left) with the inner surface of voice coil motor without being controlled to generate particles 105 during a drop test. The particles 105 then pass through the gap between the floating device and the inner surface of voice coil motor resulting from the vibration and fall on the surface of the image sensor 104 during further vibration. Therefore, a plurality of black dots or shadow regions will present on images.
In order to solve the above-mentioned drawbacks of an auto focus device with a conventional voice coil motor, a new auto focus device is thus provided to effectively solve the problems of image quality resulting from particles generated from a floating device of the voice coil motor during a drop test to improve the yield ratio of product.